Fluoride ions in zinc electrolyte cause corrosion of the aluminum cathode starting sheets commonly used in the zinc electrowinning process. Fluoride also causes the zinc which is deposited on the aluminum cathode to stick to the cathode making the removal of the deposited zinc metal difficult. The costs of replacing damaged aluminum starting sheets can be reduced by ensuring that fluoride levels in zinc electrolyte are kept low.
A known method of dealing with this problem has been to add Al.sup.3+ and PO.sub.4.sup.3- ions to the zinc sulphate solution and adjusting the pH so that fluoride is coprecipitated with the aluminum and phosphate ions, as described in U.S. Pat. No. 4,567,027. In another method, fluoride is removed by contacting the fluoride containing solution with freshly precipitated aluminum hydroxide, as described in Japanese patent application No. 8070538 dated May 26, 1980, in the name of Mitsui Petrochem Ind. Ltd.
In the known methods, the aluminum hydroxide is either precipitated in situ or is a freshly prepared precipitate which is added. The effectiveness for fluoride removal depends on the form of the aluminum hydroxide. It can be shown that the presence of amorphous Al(OH).sub.3 is needed for chemisorptive removal of fluoride. Crystalline (gibbsite) aluminum hydroxide (alumina trihydrate) is not effective for chemisorptive removal of fluoride from zinc sulphate solutions by simply stirring alumina trihydrate with the solution to be treated. It has been shown that the gelatinous aluminum hydroxide, formed by raising the pH of an aluminum salt solution, is converted on aging from an amorphous to a crystalline form. The crystalline transformation can occur within a period of from 1 to 7 days at ambient temperature when the pH is greater than 7. Therefore, the prior art methods either use a freshly prepared precipitate or form the precipitate in situ.
It has surprisingly been found by the inventors that aluminum anodizing waste treatment sludge can be used for the removal of fluoride from zinc electrolyte, even when wet (70-80% moisture) and at pH values from 6.8-9.2, and that the anodizing sludge is still effective after months of storage. The sludge is also effective if dried at 120.degree. to 140.degree. C. and crushed before use. This is surprising in view of the fact that alumina trihydrate is not effective for this purpose, as referred to above.
Aluminum anodizing operations generate a waste solution containing dissolved aluminum and acid, usually sulphuric, phosphoric and/or tartaric acid. This waste solution is usually treated with lime and/or caustic to precipitate solids, such as aluminum as Al(OH).sub.3, calcium sulphate or phosphate and other solids. Filtration of the solids gives a filter cake or waste treatment sludge, with typically 70% moisture. This is the aluminum anodizing waste treatment sludge, as referred to in this specification. The sludge contains aluminum hydroxide and other materials depending on the anodizing operation from which it is obtained.
The aluminum anodizing waste treatment sludge so produced is a waste material and the usual method of disposal of this non-hazardous sludge is landfilling, which is costly and environmentally responsible alternatives are lacking.
The present invention provides an alternative method for the disposal of the sludge which avoids the cost of landfilling and, at the same time, provides a method for the removal of fluoride from zinc electrolyte which is used in the electrowinning of zinc.